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Commit dce48a84 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

sched, timers: move calc_load() to scheduler 



Dimitri Sivanich noticed that xtime_lock is held write locked across
calc_load() which iterates over all online CPUs. That can cause long
latencies for xtime_lock readers on large SMP systems. 

The load average calculation is an rough estimate anyway so there is
no real need to protect the readers vs. the update. It's not a problem
when the avenrun array is updated while a reader copies the values.

Instead of iterating over all online CPUs let the scheduler_tick code
update the number of active tasks shortly before the avenrun update
happens. The avenrun update itself is handled by the CPU which calls
do_timer().

[ Impact: reduce xtime_lock write locked section ]

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
parent 2ff799d3

include/linux/sched.h

+1 −1
Original line number Diff line number Diff line
@@ -135,8 +135,8 @@ DECLARE_PER_CPU(unsigned long, process_counts); 
extern int nr_processes(void);

extern unsigned long nr_running(void);

extern unsigned long nr_uninterruptible(void);

extern unsigned long nr_active(void);

extern unsigned long nr_iowait(void);

extern void calc_global_load(void);



extern unsigned long get_parent_ip(unsigned long addr);

kernel/sched.c

+74 −10
Original line number Diff line number Diff line
@@ -630,6 +630,10 @@ struct rq { 
	struct list_head migration_queue;

#endif



	/* calc_load related fields */

	unsigned long calc_load_update;

	long calc_load_active;



#ifdef CONFIG_SCHED_HRTICK

#ifdef CONFIG_SMP

	int hrtick_csd_pending;
@@ -1728,6 +1732,8 @@ static void cfs_rq_set_shares(struct cfs_rq *cfs_rq, unsigned long shares) 
}

#endif



static void calc_load_account_active(struct rq *this_rq);



#include "sched_stats.h"

#include "sched_idletask.c"

#include "sched_fair.c"
@@ -2856,19 +2862,57 @@ unsigned long nr_iowait(void) 
	return sum;

}



unsigned long nr_active(void)

/* Variables and functions for calc_load */

static atomic_long_t calc_load_tasks;

static unsigned long calc_load_update;

unsigned long avenrun[3];

EXPORT_SYMBOL(avenrun);



static unsigned long

calc_load(unsigned long load, unsigned long exp, unsigned long active)

{

	unsigned long i, running = 0, uninterruptible = 0;

	load *= exp;

	load += active * (FIXED_1 - exp);

	return load >> FSHIFT;

}



	for_each_online_cpu(i) {

		running += cpu_rq(i)->nr_running;

		uninterruptible += cpu_rq(i)->nr_uninterruptible;

/*

 * calc_load - update the avenrun load estimates 10 ticks after the

 * CPUs have updated calc_load_tasks.

 */

void calc_global_load(void)

{

	unsigned long upd = calc_load_update + 10;

	long active;



	if (time_before(jiffies, upd))

		return;



	active = atomic_long_read(&calc_load_tasks);

	active = active > 0 ? active * FIXED_1 : 0;



	avenrun[0] = calc_load(avenrun[0], EXP_1, active);

	avenrun[1] = calc_load(avenrun[1], EXP_5, active);

	avenrun[2] = calc_load(avenrun[2], EXP_15, active);



	calc_load_update += LOAD_FREQ;

}



	if (unlikely((long)uninterruptible < 0))

		uninterruptible = 0;

/*

 * Either called from update_cpu_load() or from a cpu going idle

 */

static void calc_load_account_active(struct rq *this_rq)

{

	long nr_active, delta;



	return running + uninterruptible;

	nr_active = this_rq->nr_running;

	nr_active += (long) this_rq->nr_uninterruptible;



	if (nr_active != this_rq->calc_load_active) {

		delta = nr_active - this_rq->calc_load_active;

		this_rq->calc_load_active = nr_active;

		atomic_long_add(delta, &calc_load_tasks);

	}

}



/*
@@ -2899,6 +2943,11 @@ static void update_cpu_load(struct rq *this_rq) 
			new_load += scale-1;

		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;

	}



	if (time_after_eq(jiffies, this_rq->calc_load_update)) {

		this_rq->calc_load_update += LOAD_FREQ;

		calc_load_account_active(this_rq);

	}

}



#ifdef CONFIG_SMP
@@ -7091,6 +7140,14 @@ static void migrate_dead_tasks(unsigned int dead_cpu) 


	}

}



/*

 * remove the tasks which were accounted by rq from calc_load_tasks.

 */

static void calc_global_load_remove(struct rq *rq)

{

	atomic_long_sub(rq->calc_load_active, &calc_load_tasks);

}

#endif /* CONFIG_HOTPLUG_CPU */



#if defined(CONFIG_SCHED_DEBUG) && defined(CONFIG_SYSCTL)
@@ -7325,6 +7382,8 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu) 
		/* Update our root-domain */

		rq = cpu_rq(cpu);

		spin_lock_irqsave(&rq->lock, flags);

		rq->calc_load_update = calc_load_update;

		rq->calc_load_active = 0;

		if (rq->rd) {

			BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
@@ -7364,7 +7423,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu) 
		cpuset_unlock();

		migrate_nr_uninterruptible(rq);

		BUG_ON(rq->nr_running != 0);



		calc_global_load_remove(rq);

		/*

		 * No need to migrate the tasks: it was best-effort if

		 * they didn't take sched_hotcpu_mutex. Just wake up
@@ -9059,6 +9118,8 @@ void __init sched_init(void) 
		rq = cpu_rq(i);

		spin_lock_init(&rq->lock);

		rq->nr_running = 0;

		rq->calc_load_active = 0;

		rq->calc_load_update = jiffies + LOAD_FREQ;

		init_cfs_rq(&rq->cfs, rq);

		init_rt_rq(&rq->rt, rq);

#ifdef CONFIG_FAIR_GROUP_SCHED
@@ -9166,6 +9227,9 @@ void __init sched_init(void) 
	 * when this runqueue becomes "idle".

	 */

	init_idle(current, smp_processor_id());



	calc_load_update = jiffies + LOAD_FREQ;



	/*

	 * During early bootup we pretend to be a normal task:

	 */

kernel/sched_idletask.c

+2 −1
Original line number Diff line number Diff line
@@ -22,7 +22,8 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int sy 
static struct task_struct *pick_next_task_idle(struct rq *rq)

{

	schedstat_inc(rq, sched_goidle);



	/* adjust the active tasks as we might go into a long sleep */

	calc_load_account_active(rq);

	return rq->idle;

}

kernel/time/timekeeping.c

+1 −1
Original line number Diff line number Diff line
@@ -22,7 +22,7 @@ 


/*

 * This read-write spinlock protects us from races in SMP while

 * playing with xtime and avenrun.

 * playing with xtime.

 */

__cacheline_aligned_in_smp DEFINE_SEQLOCK(xtime_lock);

kernel/timer.c

+2 −52
Original line number Diff line number Diff line
@@ -1122,47 +1122,6 @@ void update_process_times(int user_tick) 
	run_posix_cpu_timers(p);

}



/*

 * Nr of active tasks - counted in fixed-point numbers

 */

static unsigned long count_active_tasks(void)

{

	return nr_active() * FIXED_1;

}



/*

 * Hmm.. Changed this, as the GNU make sources (load.c) seems to

 * imply that avenrun[] is the standard name for this kind of thing.

 * Nothing else seems to be standardized: the fractional size etc

 * all seem to differ on different machines.

 *

 * Requires xtime_lock to access.

 */

unsigned long avenrun[3];



EXPORT_SYMBOL(avenrun);



/*

 * calc_load - given tick count, update the avenrun load estimates.

 * This is called while holding a write_lock on xtime_lock.

 */

static inline void calc_load(unsigned long ticks)

{

	unsigned long active_tasks; /* fixed-point */

	static int count = LOAD_FREQ;



	count -= ticks;

	if (unlikely(count < 0)) {

		active_tasks = count_active_tasks();

		do {

			CALC_LOAD(avenrun[0], EXP_1, active_tasks);

			CALC_LOAD(avenrun[1], EXP_5, active_tasks);

			CALC_LOAD(avenrun[2], EXP_15, active_tasks);

			count += LOAD_FREQ;

		} while (count < 0);

	}

}



/*

 * This function runs timers and the timer-tq in bottom half context.

 */
@@ -1186,16 +1145,6 @@ void run_local_timers(void) 
	softlockup_tick();

}



/*

 * Called by the timer interrupt. xtime_lock must already be taken

 * by the timer IRQ!

 */

static inline void update_times(unsigned long ticks)

{

	update_wall_time();

	calc_load(ticks);

}



/*

 * The 64-bit jiffies value is not atomic - you MUST NOT read it

 * without sampling the sequence number in xtime_lock.
@@ -1205,7 +1154,8 @@ static inline void update_times(unsigned long ticks) 
void do_timer(unsigned long ticks)

{

	jiffies_64 += ticks;

	update_times(ticks);

	update_wall_time();

	calc_global_load();

}



#ifdef __ARCH_WANT_SYS_ALARM